1. Field of the Invention
The present invention relates to a rotatable connector mounted in a steering system of an automobile for use as an electrical connecting means of an air bag system and, more particularly, to a rotatable connector in which flat cables are reversely wound through a turned-back portion within a housing section defined between a stationary housing and a movable housing.
2. Description of the Prior Art
The rotatable connector has flat cables which are housed and wound between a stationary housing and a movable housing rotatably mounted on the stationary housing, and is used as an electrical connecting means of an air bag inflator or other built in a steering wheel which is mounted on a steering system of an automobile and rotated by a limited number. The flat cable is a strip-like body having a plurality of conductors. There have been known two types of flat cables: one wound in a spiral form and the other turned back halfway to an opposite direction; and the latter, or the turned-back type, can be made substantially shorter than the former.
In the turned-back rotatable connector, a single flat cable is usually employed. However, when the number of conductors increases with adoption of multiple circuits, the flat cable increases in width with the number of the conductors, thereby preventing a decrease in the thickness of the whole body of the rotatable connector. According to the rotatable connector disclosed in the specification of U.S. Pat. No. 3,763,455, however, the conductors are divided into the two flat cables to thereby meet the adoption of the multiple circuits.
FIG. 6 is a plan view schematically showing the constitution of a cable reel disclosed in the above-described patent specification. As shown in FIG. 6, a movable housing 101 having an inner cylindrical portion is rotatably connected to a stationary housing 100 having an outer cylindrical portion. A first flat cable 103 and a second flat cable 104 are inserted in an annular housing section 102 defined between the stationary housing 100 and the movable housing 101. These flat cables 103 and 104 are arranged, in opposite winding directions, in the outer cylindrical portion of the stationary housing 100 and in the inner cylindrical portion of the movable housing 101 in the housing section 102. U-shaped turned-back portions 103a and 104a are formed in a position where the winding direction is changed. The inner ends of both the flat cables 103 and 104 are connected to cable exit portions 107 and 108 disposed adjacently to the inner cylindrical portion of the movable housing 101, and led out of the movable housing 101 through the cable exit portions 107 and 108. On the other hand, the outer ends of both the flat cables 103 and 104 are connected to cable exit portions 109 and 110 located adjacently to the outer cylindrical portion of the stationary housing 100, being let out of the stationary housing 100 through the cable exit portions 109 and 110. Furthermore, in the housing section 102 are arranged a plurality of roller groups 105 and 106 along the circumferential direction thereof. The turned-back portion 103a of the first flat cable 103 is coiled around one of rollers in the roller group 105, while the turned-back portion 104a of the second flat cable 104 is coiled around one of rollers in the other roller group 106.
In the rotatable connector thus constituted, when for instance the movable housing 101 is turned clockwise in FIG. 6, the turned-back portions 103a and 104a of the flat cables 103 and 104 move clockwise within the housing section 102 by a less amount of rotation than the movable housing 101, thus winding the flat cables 103 and 104 more on the inner cylindrical portion side of the movable housing 101. Conversely, when the movable housing 101 is turned counterclockwise in FIG. 6, the turned-back portions 103a and 104a of the flat cables 103 and 104 move in the same direction by a less amount of rotation than the movable housing 101; that is, the flat cables 103 and 104 are unwound, thus being wound much on the outer cylindrical portion side of the stationary housing 100. The rollers 105 and 106 are moved in the same direction by a force from the turned-back portions 103a and 104a of the flat cables 103 and 104 at the time of winding and unwinding.
In the prior art rotatable connector described above the radial deformation of the flat cables 103 and 104 is restricted by a plurality of roller groups 105 and 106, and therefore the flat cables 103 and 104 can be smoothly moved in the circumferential direction of the housing section 102. However, since the plurality of roller groups 105 and 106 are separately arranged within the housing section 102, the roller groups 105 and 106 hit against each other, producing a sound of collision, when the rotatable connector vibrates, resulting in lowered assembling operation performance.
It is, therefore, conceivable to adopt such a structure that a moving body 111 having at least a couple of openings 111a and 111b is rotatably disposed inside the housing section 102 in place of the use of the mutually independent roller groups 105 and 106 as shown in FIG. 7, and the turned-back portions 103a and 104a of the flat cables 103 and 104 are arranged through the openings 111a and 111b. The moving body 111 is comprised of a ring-like rotary disk 112 and a plurality of roller groups 113 axially supported on the rotary disk 112, and the openings 111a and 111b are formed, equally spaced between these roller groups 113. Therefore, when the moving body 111 having the openings 111a and 111b is used, the rotary disk 112 and the roller groups 113 can be handled as one body and can prevent the collision of the roller groups 113. The aforesaid problem inherent to the prior art rotatable connector can be solved, but a later-described new problem will arise.
That is, in the above-described rotatable connector, let D be the diameter of the inner cylindrical portion of the movable housing 101 and t be the thickness of the flat cables 103 and 104, and the winding diameter of the first flat cable 103 directly wound on the inner cylindrical portion is D and the winding diameter of the second flat cable 104 wound on the inner cylindrical portion via about one turn of the first flat cable 103 becomes (D+2t). In this case, the flat cables 103 and 104 differ in winding diameter. Therefore, when the movable housing 101 is turned, the length of the flat cables 103 and 104 wound on the inner cylindrical portion, or the length of these flat cables unwound from the inner cylindrical portion, varies, with the result that the turned-back portion 104a of the second flat cable 104 having a large winding diameter moves faster than the turned-back portion 103a of the first flat cable 103 having a small winding diameter. Consequently, for instance when the flat cables 103 and 104 are fed out from the inner cylindrical portion and wound on the outer cylindrical portion as illustrated, the turned-back portion 104a of the second flat cable 104 contacts the roller 113 which is disposed at the end of the opening 111b. In this case, however, the turned-back portion 103a of the first flat cable 103 which moves at a slow speed moves only in the opening 111a and does not contact the roller 113 Therefore there arises such a problem that the moving body 111 cannot smoothly be moved by the force from the turned-back portions 103a and 104a.
The present invention, therefore, has an object to provide a rotatable connector in which a moving body disposed in a housing section has a plurality of openings through which turned-back portions of a plurality of flat cables pass, and at least one of the openings is set less in width along the peripheral direction than the other. If the openings of the moving body are all different in width, and if the opening for the turned-back portion of flat cable wound in a small diameter is set narrower than that for the turning portion of the flat cable wound in a large diameter, the turned-back portion of each flat cable can act on the moving body at the time of winding and unwinding, thereby allowing the moving body to move smoothly within the housing section.